1. Field of the Invention
The present invention generally relates to a power supply apparatus. More specifically, the present invention is directed to an improvement in efficiencies when a first-order feedback control power supply (virtual title) is driven under light load condition, and moreover, is directed to control technology capable of realizing compatibility between light load driving of the first-order feedback control power supply apparatus and a fast response thereof which constitutes the feature of the first-order feedback control power supply apparatus.
2. Description of the Related Art
As to improvements in efficiencies when power supply apparatuses are driven under light load conditions, various sorts of examples have been proposed in which these power supply apparatuses are driven in a burst mode, a voltage mode, a ripple mode, and other modes. In an example as to the above-described burst mode, a current flowing through an inductor is detected; if a power supply apparatus is driven under light load condition, then burst operations are repeatedly carried out for several cycles so as to gradually recover an output voltage of this power supply apparatus; and thus, the power supply apparatus is brought into a sleep state. At this time, since upper-sided and lower-sided power MOSFETs and an unnecessary circuit have been turned OFF, electric power is supplied to the load by utilizing energy stores in an output capacitor. As a result, although control power of the power supply apparatus is low, there is such a drawback that switching losses of the upper-sided/lower-sided power MOSFETs become large. This technical idea has been disclosed in Linear Technology Data Sheet “LTC 3410.”
Next, in an example as to the above-described voltage mode, when the power supply apparatus is driven under the light load condition, both upper-sided/lower-sided power MOSFETs are turned ON/OFF in a complementary manner only one time so as to recover an output voltage of the power supply apparatus; and at such a timing that a midpoint voltage between the upper-sided/lower-sided power MOSFETs is changed from a minus voltage to a plus voltage when the lower-sided power MOSFET is turned OFF, the power supply apparatus is brought into a sleep state. At this sleep time, only the upper-sided and lower-sided power MOSFETs are turned OFF. As a result, although switching losses may be caused by turning ON/OFF both the upper-sided/lower-sided power MOSFETs only one time, the below-mentioned drawback of high control power may occur. That is, a control circuit is normally operated in order to detect the midpoint voltages between the upper-sided and lower-sided power MOSFETs, so that the control power of this control circuit becomes high. These technical ideas have been disclosed in JP-A-2007-20315.
Also, in an example as to the above-described ripple mode, when the power supply apparatus is driven under the light load condition, both upper-sided/lower-sided power MOSFETs are repeatedly turned ON/OFF in a complementary manner so as to gradually recover an output voltage of the power supply apparatus until the output voltage of the power supply apparatus reaches from a lower limit threshold voltage of a ripple width up to an upper limit threshold voltage thereof; and at such a timing that a midpoint voltage between the upper-sided/lower-sided power MOSFETs is changed from a minus voltage to a plus voltage when the lower-sided power MOSFEET is turned OFF, the power supply apparatus is brought in to a sleep state. At this sleep time, only the upper-sided and lower-sided power MOSFETs are turned OFF. As a result, there is such a drawback that both switching losses and control power become large. These technical ideas have been disclosed in JP-A-2007-20352.